The efficiency and performance of rotary machinery, such as gas turbine engines and air compressors, are highly dependent on blade tip clearance. Specifically, blade tip clearance or tip clearance (i.e. TC) is defined herein as the distance between the tip of the rotating blade and the casing/housing within which the blade rotates. Insufficient tip clearance results in the rubbing of the blade tip against the casing/housing that the blade rotates within, and as such, can result in damage to the housing, the blade, or both. Alternatively, excessive tip clearance generally results in a significant increase in power loss and operating efficiency, such as in the case of turbines. Thus, tip clearance monitoring has become essential in detecting and predicting blade failure or structural damage suffered by the blade before it occurs.
In order to monitor blade tip clearance, several techniques have been developed. For example, capacitive methods have been developed due to their low cost and simple operational structure. However, the measured capacitance change often reflects not only the blade tip clearance variation, but also changes in the dielectric property of air that is caused due to changes in pressure and humidity of the surrounding environment. This, in turn, creates measurement inaccuracies, leading to difficulty in accurately measuring the tip clearance of a rotating blade. Alternatively, optical methods have been used to measure tip clearance with high accuracy. However, such optical methods also suffer from inaccurate measurements, which are due to debris contamination of the optical sensors used by such detection systems. Yet another tip clearance measurement technique is a microwave detection method, which is based on measuring the change in amplitude of a reflected microwave signal from a blade tip. While this technique is not affected by the presence of debris, as in the optical measurement system, the microwave detection method has difficultly performing measurements when the blade thickness is small. Another disadvantage of the optical and microwave blade tip clearance measurement methods is that in order to accommodate a sensing probe used to perform such measurements, a large through hole, which is typically larger than 10 mm in diameter, is required to be bored through the casing that encloses the rotary blades. As such, the use of such optical and microwave blade tip sensors on small-scale rotary devices, such as a turbine, is impractical.
A non-intrusive inductive blade tip clearance sensor, which is formed of 3-D solenoids that are wound around a magnetic core, has also been developed to conduct dynamic measurements of the tip clearance of rotor blades with the outside of a turbine engine casing. While this method does not require a through hole to be bored through the casing, such sensor is more sensitive to the relative vibration between the casing and the sensor. In addition, the non-intrusive sensor does not work for a casing that contains ferrous material, as such ferrous casings significantly reduce the penetrating magnetic field, and thus the output signal. Alternatively, intrusive inductive sensors have been developed and have gained considerable success for their simple structure, low cost and easy installation. However, one drawback of such inductive sensors is their low resolution. For example, such sensors cannot detect a variation in tip clearance of less than 50 um, due to the bulk size and low sensitivity of the measurement circuit utilized. Furthermore, inductive tip clearance sensors of current designs can only detect blade tip clearance at one specific location along the blade's camber line. However, during turbine engine operation, abnormal tip clearance could occur at any position along the camber line of the blade. Further, advanced health monitoring and active tip clearance control typically requires blade tip clearance measurements at multiple locations along the camber line of the blade. While multiple inductive sensors and measurement circuitries can be used to measure the dynamic tip clearance at multiple locations, implementation of such detection electronics would be complex and impractical for real-time monitoring of multiple tip clearances simultaneously.
Therefore, there is a need for a high-sensitivity inductive sensor for measuring blade tip clearances that utilizes multiple miniature-sized, spiral planar coils as sensing elements. In addition, there is a need for a high-sensitivity inductive sensor for measuring blade tip clearance that can be mounted on an inner surface of a turbine engine casing along the camber line of the turbine's rotor blade. Additionally, there is a need for a high-sensitivity inductive sensor for measuring blade tip clearance that utilizes resonance frequency division multiplexing (RFDM), which enables the simultaneous measurement of multiple, highly dynamic, blade tip clearances using only one set of measurement circuitry, with increased sensitivity.